This thing is in beta and I got into the beta to try it out. I have to say, this thing is really fun (I verified with the wotc employees that there is no NDA). It's not just for 4th ed. You can search for games in 3.5, 3rd, 2nd, even 1st editons. I haven't tried anything but 4th, but I would assume the dice mechanics are incorporated for each.
It's pretty simple yet very functional in design. It is java based, so no installation of any programs. It has a built in voice chat system that is clear.
Attack rolls and damage rolls have your bonuses added in (on the right hand side) which also makes things quick and simple. I can see that once people get the hang of it, combat would go twice, if not thrice as fast as standard PnP.
You are in control of your own character, and as you move your character around the board a blue line shows the path you took (along with the enemies and other players based on turn) to give everyone an idea of how many squares you moved and exactly how you got there.
Haven't tried DMing yet but it may be my next thing to try.